1. Field of the Invention
This invention relates to a heat stable fiber forming polyester and to a new and novel process for preparing it. More particularly, this invention relates to an improved linear high molecular weight heat stable polyester especially suitable for preparing fibers which have excellent resistance to degradation when utilized in commercial articles, such as tires, industrial belting, etc., wherein a high degree of heat is built up during use.
2. Description of the Prior Art
High molecular weight polyethylene terephthalate fiber forming polyesters are well known. They are prepared commercially either by the ester interchange reaction between dimethyl terephthalate and ethylene glycol or by the direct esterification process wherein terephthalic acid is reacted directly with ethylene glycol. These products and processes are well documented in U.S. patents such as U.S. Pat. Nos. 2,465,310; 3,050,533; 3,051,212; 3,427,287 and 3,484,410 which cover not only the basic products and processes but many improvements thereon.
Polyethylene terephthalate fibers and cords are known to exhibit excellent dimensional stability, that is, low extension or growth during service, as well as to have a high resistance to thermal degradation; however, in pneumatic tires and industrial belts under high speed conditions under heavy load, loss of tensile strength is experienced due to high temperature reactions occurring under such conditions. In efforts to remedy this problem, most research in this field has been directed to producing a high molecular weight linear polyester having a low content of free carboxyl groups. The following patents are pertinent.
U.S. Pat. No. 3,051,212 to William W. Daniels relates to reinforced rubber articles and to textile cords and fibers for reinforcing such articles. This patent discloses that a linear terephthalate polyester having a concentration of free carboxyl groups of less than 15 equivalents per million grams may be prepared in a number of different ways. One effective procedure is to treat the filaments, after they have been formed, with a chemical reagent which reacts with and "caps" the free carboxyl groups. One such agent is diazomethane.
U.S. Pat. No. 3,627,867 to Eckhard C. A. Schwarz discloses a process and apparatus for melt spinning high molecular weight polyethylene terephthalate into high-performance fibers under conditions which reduce the normally high viscosity of such polyester. Ethylene oxide or other low-boiling oxirane compound is injected under pressure into molten polyester before it is fed to the metering pump of the melt-spinning machine. The fibers are characterized by low free-carboxyl content and freedom from voids which might be expected from injection of the volatile material.
U.S. Pat. No. 3,657,191 to Rudolph Titzmann et al. is directed to a process for the manufacture of linear polyesters having an improved stability with respect to compounds with active hydrogen. Polyesters of this type are obtained by reacting polyesters with ethylene carbonates or monofunctional glycidyl ethers.
U.S. Pat. No. 3,563,847 to Grover W. Rye et al. relates to a rubber structure reinforced with a polyester reinforcing fiber modified with a polycarbonate derived from a 4,4'-dihydroxy-di(mono nuclear aryl)alkane. The polycarbonate is added to the polyester prior to fiber formation.
The closest prior art is believed to be Japanese Pat. No. 41,713 (1973) to Shima et al. on production of polyesters with low carboxy end group contents. This patent discloses reacting a polyester comprising mainly ethylene terephthalate having an intrinsic viscosity greater than 0.4 with ethylene carbonate in the presence of an organic phosphorus catalyst such as triphenylphosphine, trioctylphosphine and trimethylbenzylphosphonium benzoate. However, we have found that relatively few phosphines or phosphonium compounds are suitable as catalysts, particularly if reaction time is limited as in a continuous polyester process. As demonstrated hereinafter, the structure of the compound is very critical respecting the degree of catalytic activity. Also, in a continuous process, the timing of the reaction is very critical.
Although the above-identified patents directed to stabilized polyesters are of major interest, certain of the proposed polyester modifiers are known to be highly toxic and/or hazardous to use on commercial scale. Moreover, we have found that the others are relatively less effective in reducing the carboxyl end group concentration of the polyesters. Accordingly, we have carried out considerable research in this field to solve the long-standing problem of producing a high molecular weight polyester stabilized against deterioration under high temperature operating conditions.